1. Field of the Invention
The present invention relates to a touch sensor, and more particularly to a method for measuring speed of a conductor slipping through a capacitive sensor.
2. Description of the Related Art
In recent years, due to the development of technology, control buttons, such as buttons of an elevator or a game console, evolve from a mechanical type of button into a touch sensor. FIG. 1 is a circuit diagram depicting a capacitive touch sensor in the prior art. Referring to FIG. 1, the touch sensor includes a sensing electrode 101, a resistor 102 and a sensing-control terminal 103, wherein the sensing electrode 101 in the circuit is equivalent to a grounding capacitor Cx.
FIG. 2 illustrates an operational waveform diagram of a node A coupled to the sensing electrode 101 and the resistor 102. Referring to FIGS. 1 and 2, the sensing-control terminal 103 charges the node A to a first preset voltage V20 at the beginning, and then the node A is set to high-impedance. Afterward, since the sensing electrode 101 is equivalent to the grounding capacitor Cx, the sensing electrode 101 starts to discharge through the resistor 102. The sensing-control terminal 103 continuously detects a voltage of node A. When the voltage of node A discharges to a second preset voltage V21, the sensing-control terminal 103 determines whether a finger touches the sensing electrode according to a period when the voltage of node A is discharged from the first preset voltage V20 to the second preset voltage, and then the sensing-control terminal 103 begins to charge the node A.
Referring to FIG. 2, the waveform 201 is a voltage waveform of node A when the finger does not touch the sensing electrode 101, and the waveform 202 is a voltage waveform of node A when the finger touches the sensing electrode 101. According to the waveforms, when the finger touches the sensing electrode 101, the equivalent capacitor of the sensing electrode 101 is increased so that a discharge time T2 of the waveform 202 is longer than a discharge time T1 of the waveform 201. Therefore, as long as it is determined that the period when the voltage of node A is discharged from the first preset voltage V20 to the second preset voltage V21 is longer than the discharge time T1 by the sensing-control terminal, it can be determined that the sensing electrode 101 is touched.
In a specific application, especially a game console, it may be necessary to sense the speed of a conductor slipping through a capacitive sensor. FIG. 3 illustrates a waveform depicting the variation of the equivalent capacitor of the sensing electrode 101 when a conductor slips through the sensing electrode. Referring to FIG. 3, a conventional method for measuring the speed of a conductor (such as finger) moving through the sensing electrode 101 is to determine the speed according to a period from the time when the conductor is connected with the sensing electrode 101 to the time when the conductor is disconnected from the sensing electrode 101. The abovementioned period is determined according to the variation of the equivalent capacitance Cx of the sensing electrode 101 with respect to the time. Generally speaking, the equivalent capacitance Cx can be obtained by the variation of Cx with respect to the time. The conventional method includes:
the first step of presetting a first threshold capacitance CT1 (the higher threshold value) and a second threshold capacitance CT2 (the lower threshold value);
the second step of determining whether the estimated value of the equivalent capacitance Cx is in excess of the first threshold capacitance CT1;
the third step of starting to count the time when the equivalent capacitance Cx is in excess of the first threshold capacitance CT1; and
the fourth step of stopping to count the time when the equivalent capacitance Cx is lower than the second threshold capacitance CT2, and then determining the time when the conductor slips through the sensing electrode 101 according to the counted time.
However, if the human finger is used, for example, each person has a different finger condition, and thus the different finger pressure and the different contact area between the human finger and the sensing electrode 101. FIG. 4 illustrates a waveform depicting the variation of the equivalent capacitance of the sensing electrode 101 when the finger slightly slips through the sensing electrode 101. Referring to FIG. 4, when the finger slightly slips through the sensing electrode 101, the finger and the sensing electrode do not tightly contact with each other. Thus, the variation of the equivalent capacitance Cx is relatively small. If the equivalent capacitance Cx is just smaller than the first threshold capacitance CT1, the abovementioned third step cannot be triggered. In addition, even if the equivalent capacitance Cx is just larger than the first threshold capacitance CT1, the measured time may not be precise since the equivalent capacitance Cx is too small.
In addition, the surface material of the sensing electrode 101 is generally made of plastics, such as polyethylene, polypropylene, and so on. In this type of the capacitive sensor, the sensing electrode 101 tends to be comparatively influenced by the electrostatic charges on its surface material. This kind of plastics, such as polyethylene, polypropylene, and so on, has a characteristic that the accumulated electrostatic charges therein are hard to be eliminated. FIG. 5 illustrates a waveform depicting the variation of the equivalent capacitance of the sensing electrode 101 when the sensing electrode 101 is affected by the electrostatic charges. Referring to FIG. 5, when the finger operates on the abovementioned plastics, the electrostatic charges will be gradually induced into or out of the plastics so that the electrical field of the surface of the sensing electrode 101 will be changed. As the electrostatic charges on the surface of the sensing electrode 101 change, the waveform of the equivalent capacitance may vary in a manner similar to the variations of the waveforms 51 or 52. Therefore, the measured time Tn may be influenced and becomes Ts or T1.
Hence, the inaccuracy is induced as long as the speed of a conductor slipping through the sensing electrode is determined according to the simple settings of the first threshold capacitance CT1 and the second threshold capacitance CT2.